Geology of Dorset

Dorset/ˈdɔrsɨt/ (or archaically, Dorsetshire) is a county in South West England on the English Channel coast. Covering an area of 2,653 square kilometres (1,024 sq mi); it borders Devon to the west, Somerset to the north-west, Wiltshire to the north-east, and Hampshire to the east. The great variation in its landscape owes much to the underlying geology which includes an almost unbroken sequence of rocks from 200 Ma to 40 Ma and superficial deposits from 2 Ma to the present.[1] In general the oldest rocks (Early Jurassic) appear in the far west of the county, with the most recent (Eocene) in the far east. Jurassic rocks also underlie the Blackmore Vale and comprise much of the coastal cliff in the west and south of the county; and although younger Cretaceous rocks crown some of the highpoints in the west, they are mainly to be found in the centre and east of the county.[2]

Dorset's coastline is one of the most visited and studied coastlines in the world because it shows, along the course of 95 miles (153 km) (including some of east Devon), rocks from the beginning of Triassic, through the Jurassic and up to the end of the Cretaceous, documenting the entire Mesozoic era with well-preserved fossils.[3] Throughout Dorset there are a number of limestone ridges.[4] The largest and most notable is the band of Cretaceouschalk that runs from the south-west to the north-east of the county and forms part of the Chalk Group that underlies much of the south of England, including Salisbury Plain, the Isle of Wight and the South Downs. Between the bands of limestone and chalk are wide clayvales with flood plains.

South-east Dorset, around Poole, Bournemouth and the New Forest, lies on younger and less resistant beds: Eocene clays (mainly London Clay), sands and gravels. These rocks produce thin soils that historically have supported a heathland habitat. The chalk and limestone hills of the Purbecks lie atop Britain's largest onshore oil field. The field, operated from Wytch Farm, produces a high-quality oil and has the world's oldest continuously pumping well at Kimmeridge, which has been in use since the early 1960s.[5][6] The source of this oil is the organic-rich shales found in the lower lias. Landslides along the coast have been known to ignite these shales causing cliff fires, the most recent of which occurred in 2000.[7][8]

380 million years ago (Ma) the landmass that was later to form Southern Britain was some 19 degrees south of the equator and lay on the northern shore of an ocean basin that separated the continents of Laurasia and Gondwanaland. When these continents collided to form the single super-continent of Pangaea, the sediments on the ocean floor were pushed up and over while the molten rock below the surface was forced out.[9] Today, these igneous intrusions and the red sandstones, visible in the neighbouring county of Devon, slope away from west to east and are deeply buried beneath younger deposits in Dorset.[10] Around 204 Ma Dorset, now 30 degrees north, was under water and the first ammonites (Psiloceras planorbis) appear among the shales and limestones that make up the lower Lias.[11]

Formed somewhere between 191 Ma and 204 Ma, in what was then a shallow marine environment, the lower Lias is composed of Blue Lias, Black Ven Marls and Green Ammonite Beds. Mostly covered by lush vegetation, it forms the floor of Marshwood Vale in the west of Dorset and can be seen in stream beds, where the land has been excavated, and along the coast to the west of Seatown. The sides of the vale are mainly made from the clays and sands of the upper and lower Lias while younger strata from the Cretaceous Period (145-66 Ma), crown the higher points.[12] The Blue Lias is the lowest of the Liassic stratum and where it is visible on the coast near Lyme Regis is layered with hard limestone and oil-rich shale.[13] The iron pyrites in the clay can heat up when exposed to the air and occasionally ignite the shales.[14]

Around Lyme Regis, during the 18th and 19th centuries, the collecting and sale of fossils became a popular occupation. Landslides and the excavation of the clays, used in cement production, exposed not only an abundance of ammonites of varying size, but much larger specimens such as ichthyosaurs and plesiosaurs.[14] Although these rocks were formed underwater, the discovery of fossil wood, land-dwelling animals and a pterosaur suggest that dry land was close by.[15]

Laid down between 191 Ma and 190 Ma years ago in a shallower marine environment that was subjected to much more turbulent weather conditions, the sediments of the middle Lias are not as muddy as those in the lower Lias, and are much nearer to silt and sand.[16] These sands are thought to have come from the islands that now form part of Cornwall and South Wales.[16] Again, inland exposure is poor, although the middle Lias is visible along the spring line of the hills that surround the Marshwood Vale. The cliffs either side of Eype and the coastal stretch between Thorncombe and Watton Cliff provide the best view.[16] The base of the middle Lias is composed of three thick layers of calcareous sandstone beds separated by marls. These resistant bands form massive buttresses along the sea-cliffs and, where eroded, boulder aprons on the foreshore.

Above these three layers is the Eype clay, which was probably deposited in deeper, calmer waters.[17] Ammonites found in this layer have apparently been attacked by larger crustaceans. The fossilised remains of large numbers of brittle stars, found towards the end of this deposition, indicate that they were rapidly covered. Theories put forward suggest a great storm or tidal wave was the cause and indeed many of the remains appear to have been swept along the sediment, some losing limbs on the way. This particular section of the lias is appropriately known as the "starfish bed".[18] The succeeding layers of Downcliffe and Thorncombe sands also appear to have been deposited in a periodically stormy environment where silts and sands dominate. Pebbles and cobbles are found here, encrusted with fossilised animals, which suggests that some of the sediments were already lithified; there is also some evidence of localised fault activity.[19] The last layer in the middle Lias is the Marlstone Rock Bed.

The lowest part of the upper lias, formed around 190 Ma, is known as the junction bed and despite representing 3,000,000 years, is only 0.6 to 1.5 metres thick, where it can be seen at Eype Mouth.[20] In Yorkshire the equivalent strata is around 100 metres thick, suggesting that Dorset remained underwater much longer and was robbed of supplies of sand and clay. This bed is composed of fine grain limestones from white to pale pink in colour.[20] The clearest evidence of fault activity exists in a section of cliff between West Bay and Eype Mouth, where the junction bed thickens in a section of steps, and fractures in the rock contain sediment from the ocean bed which, most likely, was sucked in as the cracks opened.[20]

The remaining part of the upper Lias is the bright yellow Bridport sandstone that gives the cliffs between West Bay and Burton Bradstock their distinctive colour.[21] The sands form a band that runs inland to Yeovil. In the east of the county, these sands are some 900 metres below the surface and form the Wytch Farm oilfield’s middle reservoir.[21]

The succeeding Inferior Oolite was deposited in a shallow, tropical marine environment, 35 degrees north of the equator. It is only 2 metres thick in the south but increases to 20 metres in the north, near Sherborne.[22] It contains many fossils, including bryozoans, brachiopods, ammonites, belemnites, gastropods, bivalves and echinoids, and like the previous middle and upper Lias shows signs of fault-controlled deposition.[22] It appears in some places that the sea floor was breached and sediments and fossils were trapped like those in the junction bed between West Bay and Eype. Most of Dorset was starved of sediment and condensed limestones collected.[22] These limestone beds are often iron rich, lending them a rusty colour. One particular bed, exposed at Burton Cliff, contains large numbers of orange, discus-shaped concretions, which on closer inspection reveal themselves to be pieces of shell from a species of mussel, coated with a thin layer of iron-rich sediment.[23]

In the upper section of the Inferior Oolite there are two beds packed with numerous types of sponge, which can be seen around Shipton Gorge and Burton Bradstock.[24] Despite being formed in a marine environment, the oolite, like the lower Lias, contains some fossilised remains of land-dwelling creatures, including two species of Megalosaurus that were found near Sherborne.[24]

Fuller's Earth Formation clays were deposited across much of southern Dorset when the continental shelf subsided, forcing the region deeper underwater. The lower clays are visible at Burton Bradstock and contain large numbers of Bositra buchi. It has been suggested that these bivalves were able to swim freely and thus able to avoid the soupy mud on the sea floor.[25] A limestone known as Fuller's Earth Rock is found in the north-west of the county, between the clays south and east of Sherborne. It is rich in ammonites and bivalves.[26] Previously referred to as the Upper Fuller's Earth Clay, the succeeding Frome Clay can be seen to the west of West Bay where it forms the major part of the West Cliff. It contains many Brachiopods, particularly in the Weymouth Anticline where the oyster beds are 5 metres thick.[26] At the junction between the Frome Clay and the overlying Forest Marble is the Boueti Bed, so called because of the large numbers of the brachiopod, Goniorhynchia boueti found there. It is best viewed on the Herbury peninsula south of Langton Herring.[26] The limestone known as Forest Marble is not a true metamorphic marble but it takes a high polish, and has been used as a building material and marble substitute for many years.[27] Outcrops of Forest Marble are concentrated around the Weymouth Anticline, the coastal escarpment between Burton Bradstock and Abbotsbury, and inland as far as Bothenhampton, where they disappear below younger Cretaceous deposits. Surfacing once more at Rampisham, they turn east and then north into Somerset.[28] The limestone does not form one continuous bed as previously thought, but rather forms pockets in a formation which is predominately clay. It is likely that severe tropical storms swept up large quantities of shells and other invertebrates and deposited them in this way.[28] The formation of the Forest Marble suggests a shallowing of the sea and the remains of pieces of tree, and bits of land-dwelling animals, are found among the turtles, frogs and salamanders within.[28]

Between 160 Ma and 145 Ma, the future Dorset, at 36 degrees north, still had a tropical climate and the limestones, clays, silts and sands from this period were laid down in a marine environment that ranged from deep water to tidal shallows.[29] Above the Forest Marble lies the Cornbrash sequence which is dominated by rubbly limestone. Unsuitable for building stone, these limestones have been used lime production and road construction. In the north of the county an already lithified Cornbrash floor was being eroded, and the resulting pebbles gathered serpulid worms, bryozoans and bivalves as they rolled around beneath the shallow sea.[29] The shallow water in which these limestones were deposited was followed by marginally deeper water in which the Kellaway Beds were laid down. The Kellaway Beds are only visible in the banks of streams and rivers, and the brick pits at Chickerell and Rampisham where the succeeding Oxford Clays were quarried.[30] The Oxford Clays were deposited in much deeper water and cover the valley floors between Weymouth and the Blackmore Vale.They can also be seen in the cliffs at Tidmoor Point, Jordan Hill and the previously mentioned brick pits at Chickerell.[31] Poor exposure, coupled with a decline in the brick industry has made detailed mapping of the clays difficult which now relies heavily on trenches dug for services. These excavations have unearthed some impressive examples of Septarian Nodules.[31]

The exposure of Nothe Grit, to the south of Weymouth, suggests a return to shallower water conditions and a more turbulent environment. In the following 4.5 million years a cycle of lagoonal, tidal and deep water environments caused by a world-wide fluctuations in sea levels, deposited beds of sand, clay and limestone across the entire county.[32] The Corallian strata can be seen on the Weymouth anticline, between Abbotsbury and Osmington. It then disappears to emerge as a low escarpment between Mappowder and Cucklington on the Somerset border. Near the top of this formation is the Ringstead Waxy Clay in which large delta shaped oysters are found and at the top of the Corallian is the Ringstead Coral Bed.[33]

Benecliff grit, a soft sandstone containing large doggers, is well exposed at the headland, Bran Point. Once a reservoir, oil sometimes seeps from the cliff here, and an oily residue is present in the sands nearby.[33]

The varied rock types of the Corallian are superseded by the Kimmeridge clay, a bed of dark clays and shales up to 500 metres thick. These deposits occurred in a deep marine environment, 38 degrees north, 140 Ma to 145 Ma.[34] Deposited in an oxygen deficient environment, the Kimmeridge beds make up the major part of the Purbeck Monocline and the periphery of the Weymouth Anticline. They form a low lying band running north to south through the Blackmore Vale, and underpin the strip of land between the isle of Portland and the mainland.[34][35] The unusual iron rich sediment near Abbotsbury was, in a departure from the norm, deposited in shallow iron rich water and the pale bands throughout the formation were caused by Coccoliths following periodic blooms of algae.

Many of the fossilised remains are squashed flat, indicating that the Kimmeridge beds were heavily compacted, perhaps to an eighth of their original thickness. This was not sufficient to produce oil reservoirs like those beneath the North Sea, yet the oil rich shales have, in the past, been economically important as a local fuel.[36]

It is unclear whether the formations created between the Upper Jurassic and Lower Cretaceous, were deposited across the whole of Dorset like the preceding beds, and subsequently eroded, or only formed in the areas in which they are now found.[37]

The Portland sands were created in a deep marine environment whereas the Portland stone was a near shore deposit.[37] The lower Portland Sand formation, usually greyish blue in colour, is a dolomitic sand limestone best viewed on the West Weares, Gad Cliff and Hounstout. The overlying Portland stone is a distinctive white oolite containing thick shelled fossils and marine algae.[38] The marine algae which formed large reefs, is responsible for the occasional stacked and arched bands in the formation and is best viewed in the Roach, an extremely fossiliferous limestone confined to the north of Isle of Portland.[39]

To the north of the oolite is a fine grained limestone, which was deposited in calm, shallow water. It contains flint-like rocks known as chert and if it wasn't for the presence of very large ammonites, might be mistaken for an early chalk.[40] Previously thought to be around the middle of the Purbeck Strata, modern research suggests that the end of the Portland limestone marks the boundary between the Jurassic and Cretaceous periods.[41]

The Purbeck Limestone Group is a succession of limestones, shales and clays named after the Isle of Purbeck, laid down mainly in very shallow lagoons and freshwater lakes, with only occasional marine influence at the lower levels where halite, gypsum and other evaporites are found.[41] The muddled sediments around Lulworth are thought to be caused by a thick layer of these evaporites dissolving. There is also evidence of uplift and erosion of earlier formations. The climate around this time had changed from a tropical to a sub-tropical one, with pronounced wet and dry seasons as indicated by the growth rings in the fosslised trees also found in the lower levels of the Purbeck strata.[42] These trees grew around the low coastal areas and were, from time to time, subjected to drowning from the sea. The fallen trees and their stumps became covered by stromatolites which trapped sediments. The limestones at this level, which vary greatly in colour, are composed of densely packed shells of the freshwater snail Viviparus.[43][44] Their ability to take a very high polish has led them to become known as Purbeck Marble. Like the older Forest Marble it is a sedimentary marble rather than a true metamorphic one.[45]

The Purbeck Limestone Group has yielded a diverse selection of fossilised remains, including rare reptiles, amphibians and mammals, for which the formation has become internationally important.[46]

The Wealden Beds are confined to the south and increase in thickness from 107 metres to 716 metres, between Upwey and Swanage.[47] By 130 million years ago, the waters covering Dorset's landmass had subsided leaving the area almost completely terrestrial save for the large rivers that dominated it.[47] These rivers drained the upland areas to the north and west, and were responsible for depositing much of the succeeding sediment across southern Dorset.[47] Comprising clays, silts, sands and grits, they cover the floors of the valleys between Lulworth and Swanage. The sands and grits are also exposed in the cliffs there, and form ridges along the valleys.[47] Traces of granite from Cornubia, the landmass that later became Cornwall, were also washed down.[48] The Wealden Beds contain a great deal of plant matter and charcoal indicating periodic fires in the region. At Mupe Bay, some of the sands have been bound together by oil and formed into sandstone boulders, suggesting that oil production had already begun when these sands were deposited.[48]

The marginal marine incursions that occurred in the previous period increased in severity until Dorset was completely underwater. The geology from this period indicates a gradual flooding of the county from the south-east to the north-west.[49] The earliest deposit, the Lower Greensand, covers the preceding Wealden Beds in the south, and the Oxford and Kimmeridge clays in the north.[49] It is so called because of its green colour which is caused by the presence of the mineral glauconite.[50]

In the west, overlying the eroded Lias, is a thin layer of Gault Clay which decreases in depth from 27 metres to 7 metres the further west it stretches, and can be located by the spring lines around the chalk ridges and escarpments.[51] The water that sits between these two beds is responsible for the many landslips which have occurred throughout the county's history.[50] Around Compton Valence, it lies on the Fuller's earth whereas between Ringstead and White Nothe, it covers the Kimmeridge Clay and the Portland and Purbeck stratas.[52] Because geologists are certain all the formations from the Lias to the Kimmeridge Clay were deposited over the whole of the county, the Gault clay reveals the scale of uplift and erosion which must have taken place before Dorset was flooded.[53] The overlying Gault is revealed in the spring lines around the chalk ridges and escarpments.[53] This water is also responsible for the many landslips throughout Dorset's history from the prehistoric to modern times.[50]

Above the Gault is the Upper Greensand, which varies from the golden sands that crown the West Dorset heights, such as Pilsdon Pen and Golden Cap, to the rubbly sandstones found in the centre of the county.[50] The fossiliferous, green sandstone found around Shaftesbury contains many ammonites which, when compared to other deposits, indicate condensed deposition during different periods at different locations and detailed analysis reveals that basement structures were still active at this time.[50]

The sea levels remained more or less unchanged until the end of the Cretaceous period and despite Dorset having moved even further north to 40 degrees, the climate had reverted to a tropical one. During this period, the sea deposited an enormous amount of chalk across the county, perhaps as much as 300 metres more than there is currently.[54] The first chalk deposits contained sands and grits thought to have originated in Cornubia. This gritty chalk was deposited across central Dorset and can clearly be seen in the sides of Eggardon Hill. As the water deepened, plankton, coccoliths and foraminifera became the main source for the calcereous soup which would later form the Dorset chalk.[54] The chalk forms a narrow strip inland between Studland and Worbarrow Bay when it reaches the coast once more. A much wider strip continues westward along the coast until it meets the wide swathe of chalk that cuts north-west across the county and forms the south-east point of the Hampshire Basin.[55] Generally the Dorset chalk is too soft to be used as a building stone except along the Ridgeway where faulting and folding produced a particularly hard variety popularly used by the Romans in their mosaics. The chalk often contains larger fossils including ammonites, belemnites and brachiopods. Flint occurs throughout in both nodule and tabular layers.[55]

The iridium rich layer of sediment, found around the world at the K-Pg boundary, is missing in Dorset. This may be due to the uplift and erosion that removed the upper layers of the chalk.[56] The sediments resting on the Dorset chalk are, for the same reason, distinctly different from those in other parts of the world, and the remains of the plants and animals within, much more recent.[57]

The expansion of the North Atlantic, which first occurred during the Triassic period, now reached a point where it was affecting the Dorset landscape. Areas that were previously underwater were exposed to erosion as the chalk was folded and forced upwards.[58] The early deposits from this period, found across South-East England, do not occur in Dorset. The small amounts found in Devon however, coupled with the many solution holes found across the Dorset chalk, suggests that erosion may have removed them.[59] Some of the larger solution holes, known as dolines, are found on the heaths between Dorchester and Bere Regis. Near Briantspuddle is Culpepper's Dish, 86 metres wide and over 21 metres deep.[60]

The first deposits found from this period in Dorset are the Reading Formation clays. A sub-division of the Lambeth Group, they are visible above the chalk at Studland. Mottled, brightly coloured and generally lacking fossils, the clays are thought to be deposited beneath lagoons.[60] In the east of the county, however, fossilised remains of gastropods and sharks' teeth have been found alongside flint pebbles, clearly identifying these clays as marine deposits.[61]

The Bracklesham Group, found across much of the Dorset portion of the Hampshire Basin, were mainly deposited by rivers and form the foundations of the Dorset heathland. The frequent and heavy rains which occurred during this period transported large amounts of material from northern uplands across the Purbeck and Weymouth areas.[61] A thick layer of Chert gravel from the Purbeck Strata can be found on Blackdown Hill near Portesham, but it is not known whether it was carried there or whether it is the remains of an earlier deposit.[62] Blocks of Quartzite can be found across the downlands of Southern Dorset and these sarsons have been used in the some of the local prehistoric monuments.[61]

To the north and east of the county, much finer sands and gravels are found, including ball clay which was widely mined on the heaths around Wareham.[63] The fine particles which make up the clay are believed to have originated in the west where rocks were being broken down by the alternating dry, humid conditions and torrential rain, which in turn formed rivers and carried the material to its current position.[63] Overlying the clay is the Agglestone grit, an iron cemented sandstone which forms the Agglestone near Studland, and has been used in the buildings in and around the heathland where it is found.[64]

The crumpled rock strata at Stair Hole caused by a collision between the African and Eurasian plates

Fossilised palms taken from the cliffs at Bournemouth confirm that Dorset had a tropical climate at the time.[64]Creech Barrow Hill is crowned with a unique early Eocene limestone that has yielded some important fossils. Outcrops of rock from the Barton Group occur along the coast between Bournemouth and Hengistbury Head.[65]

During this period, Dorset drifted a further 9 degrees north, the climate cooled rapidly and by 37 Ma was temperate.[66] Mainly terrestrial with fresh and brackish water environments, the county would, despite its low-lying nature, remain largely without marine influences except for some minor incursions at the end of the ice ages. The late Palaeogene deposits that filled the Hampshire Basin as the sea levels fell are not found in Dorset, nor are the Neogene, which are absent from virtually all of the British Isles.[66]

The collision of the African and Eurasian plates that occurred around this time caused a violent uplift in the south of the county, the results of which can be seen clearly at Stair Hole, where the strata run almost vertically.[67] Further, but less dramatic folding and uplift, along the county's subsurface faults, coupled with continuing erosion of Jurassic and Cretaceous sediments, created Dorset's modern landscape and drainage patterns.[68]

During the last 1,500,000 years, Britain was subjected to a number of extreme fluctuations in temperature which included three ice ages. Dorset was not affected by glaciation but did experience very low temperatures with permafrost on the high ground.[69][70] Sudden changes in sea-level, and successive warming and cooling, took their toll on the landscape.[71] The two raised beaches at Portland created 210,000 and 125,000 years ago are a dramatic testament to the fluctuations in sea levels. The cause of these two fluctuations was twofold: the growth of the ice, which absorbed large quantities of water, and the weight of the ice in the north that caused the landmass to tilt.[72]

During the summer months, partial melting of the ice created large rivers that washed enormous amounts of sand and gravel across the county, forming huge terraces in both the upland areas and valleys.[72] These rivers cut down into the landscape as they made their way to the lower sea levels. This action caused many large-scale landslips, most notably around Shaftesbury and Abbotsbury.[72]

Landslips caused by marine erosion still occur today, with some of the largest taking place along the West Dorset Coast. Fossilised remains, including the teeth and tusks of elephants and mammoths dating back 500,000 years, are often exposed.[73] Flint and Chert tools from early human visitors have been retrieved from the river terraces around the Dorset and Devon border, although these may have been transported here with the other river deposits.[73]Chesil Bank a 29 kilometres (18 mi) long barrier beach stretching from West Bay to Portland, was created 10,000 years ago, when sea levels rose at the end of the last ice age.

The Variscan orogeny is the oldest event for which there is evidence in Dorset, forming a major series of thrust faults in Carboniferous and older rocks. These thrusts trend west–east, dip moderately towards the south and have controlled later basin development and subsequent phases of inversion. During the Permo-Triassic the area was affected by a phase of rifting that reactivated the older thrusts in extension.[74] These extensional faults were themselves reactivated during further rifting in Jurassic to early Cretaceous times. The area experienced the effects of the early stages of Alpine collision during the Late Cretaceous to Palaeogene, with reverse reactivation of many of the extensional faults, creating the most visible of the structures in the Dorset area, such as the Purbeck Monocline.

The Litton Cheney Fault is a west–east-trending, south-dipping normal fault. The fault was active during deposition of the Lias Group and again during the Late Jurassic to Early Cretaceous. There is evidence of minor reversal on the fault during the Late Cretaceous to Palaeogene inversion.[75]

This major west–east-trending, south-dipping normal fault was mainly active during the Late Jurassic to Early Cretaceous, as shown by the local preservation of the Wealden in its hanging wall. It was reactivated as a reverse fault during the Late Cretaceous to Palaeogene, and now juxtaposes Portland and Purbeck beds in the hanging wall against Chalk in the footwall. The fault is not directly linked to faults beneath the Triassic salt layer.[76]

The Weymouth Anticline has a west–east trending axis and plunges to the east. Strata beneath the Upper Triassic halite are unfolded indicating that the salt is acting as a decollement. The anticline is a result of inversion along the Abbotsbury–Ridgeway Fault, although it has been suggested that it developed by the tightening of an earlier rollover anticline, formed during an earlier extensional phase, due to the soft-linked nature of the faulting.[76]

This monoclinal structure forms part of a semi-continuous structure extending eastwards through the Isle of Wight as the Portland-Wight Monocline (or Disturbance). This was formed by reverse reactivation of the Purbeck–Isle of Wight fault zone.[77]

^"The Chalk of Dorset"(PDF). Baseline chemistry of selected reference aquifers in England and Wales. British Geological Survey/Environment Agency. 2002. p. 15. Archived from the original on 9 July 2012. Retrieved 24 January 2012.